Our investigation into the morphogenesis of bacteriophage T4 will be continued, with emphasis on two aspects: (1) The detailed mechanisms of phage tail fiber assembly and attachment, and (2) the role of the host cell in the viral assembly process. (1) The assembly and attachment of T4 tail fibers requires several proteins, under phage genetic control, which are not found in the complete virus particle. Biochemical and genetic approaches will be followed in an attempt to understand the mechanisms by which these proteins promote or direct the assembly of the structural protein components of the tail fibers. (2) Bacterial mutants have been isolated in which normal T4 assembly is blocked, indicating that host factors play a necessary role in viral morphogenesis. Such mutants will be characterized genetically, physiologically, and by electron microscopy, to determine the stages at which phage assembly can be blocked by host mutation, and the nature of the host factors involved in normal assembly.